The present invention relates generally to the field of optical systems. More particularly, the present invention relates to a method and apparatus for providing high power laser outputs useful for industrial applications such as trimming, marking, cutting, and welding. Merely by way of example, the invention has been applied to a relay fiber including a rare earth dopant optically coupled to an output of a fiber amplifier. However, the present invention has broader applicability and can be applied to other optical components including optical circulators, optical isolators, end caps, pump combiners, optical modulators, optical switching elements, wavelength-division multiplexing (WDM) elements, fiber gratings, beam shaping elements, optical taps, Diffractive Optical Elements (DOE), and the like.
Conventional laser-based material processing has generally used high peak power pulsed lasers, for example, Q-switched Nd:YAG lasers operating at 1064 nm, for marking, engraving, micro-machining, and cutting applications. More recently, laser systems based on fiber gain media have been developed. In some of these fiber-based laser systems, fiber amplifiers are utilized.
Some optical amplifiers and lasers utilizing a fiber gain medium are optically pumped, often by using semiconductor pump lasers. The fiber gain medium is typically made of silica glass doped with rare-earth elements. The choice of the rare-earth elements and the composition of the fiber gain medium depend on the particular application. One such rare-earth element is ytterbium, which is used for optical amplifiers and lasers emitting in the 1020 nm-1100 nm range. Another rare-earth element used in some fiber gain media is erbium, which is used for optical amplifiers and lasers emitting in the 1530 nm-1560 nm range.
The wavelength of the optical pump source used for ytterbium-doped fiber amplifiers and lasers is typically in the wavelength range of 910 nm to 980 nm. The wavelength of the optical pump source used for erbium-doped fiber amplifiers and lasers is typically in a wavelength range centered at about 980 nm or about 1480 nm.
When a fiber laser or amplifier is operated in a high power mode, variations in the electric field of the light beam in the optical fiber produce acoustic vibrations in the fiber via electrostriction. This acousto-optic interaction between light and acoustic phonons in the fiber, referred to as Stimulated Brillouin Scattering (SBS), results in an interference pattern that feeds a coherent traveling acoustic wave. This wave eventually becomes highly reflective and substantially degrades system performance. SBS tends to limit the power output available from fiber amplifier and laser systems. Thus, there is a need in the art for improved methods and systems to increase the output power of optically active fiber systems.